Lecture 7.1: Control of Infectious Diseases

Question 1

Potential of an infectious agent to spread in a
population is dependent on 4 factors, name them

Answer 1

• Probability of transmission in a contact between an.
  infected individual and a susceptible one
• Frequency of contacts in the population - contact
  patterns in a society
• Duration of infectiousness
• Proportion of the population/contacts that are already
  immune, not susceptible

Question 2

What does a R0<1 mean?

Answer 2

The disease will disappear

Question 3

What is the Basic Reproductive Rate (R0)?

Answer 3

Average number of individuals directly infected by an infectious case (secondary cases) during his or her entire infectious period, when she or he enters a totally susceptible population

Question 4

What does a R0=1 mean?

Answer 4

The disease will become endemic

Question 5

What does a R0> 1 mean?

Answer 5

There will be an epidemic

Question 6

What is an Epidemic?

Answer 6

It is the rapid spread of disease to a large number of hosts in a given population within a short period of time

Question 7

What does it mean when a disease is Endemic?

Answer 7

Regularly occurring within an area or community

Question 8

R0 is Affected By…? (3)

Answer 8

• The frequency of contacts in the host population
• The probability of infection being transmitted during
  contact
• The duration of infectiousness

Question 9

Basic formula for the actual value of R0?

Answer 9

R0= β * κ * D

Question 10

What does β stand for in R0= β * κ * D? What can affect β?

Answer 10

• Risk of transmission per contact (i.e. attack rate)
• Condoms, face masks, hand washing –> β ↓

Question 11

What does κ stand for in R0= β * κ * D? What can affect κ?

Answer 11

• Average number of contacts per time unit
• Isolation, closing schools, public campaigns –> κ ↓

Question 12

What does D stand for in R0= β * κ * D? What can affect D?

Answer 12

• Duration of infectiousness measured by the same time
  units as κ
• Specific for an infectious disease
• Early diagnosis and treatment, screening, contact tracing
  –> D ↓

Question 13

What is the Effective Reproductive Number (R)?

Answer 13

It is the expected number of new infections caused by an infectious individual in a population where some individuals may no longer be susceptible

Question 14

What is Immunisation?

Answer 14

• Immunisation is the process whereby a person is
  made immune or resistant to an infectious disease,
  typically by the administration of a vaccine
• Vaccines stimulate the body’s own immune system to
  protect the person against subsequent infection or
  disease

Question 15

What is Herd Immunity?

Answer 15

• Level of immunity in a population which prevents.
  epidemics even if some transmission may still occur
• Presence of immune individuals protects those who
  are not themselves immune

Question 16

What is the Herd Immunity Threshold?

Answer 16

Minimum proportion (p) of population that needs to be immunised in order to obtain herd immunity

Question 17

How to calculate the Herd Immunity Threshold?

Answer 17

p = 1 - 1/R0

Question 18

What is an Outbreak?

Answer 18

Occurrence of more cases of disease than expected in a
given area, among a specific group of people, over a particular period of time

Question 19

Why Investigate Outbreaks? (8)

Answer 19

• Stop the outbreak
• Find and neutralise the source (cause)
• Prevent additional cases
• Prevent future outbreaks
• Improve surveillance and outbreak detection
• Improve our knowledge
• Keep the public’s confidence
• Training

Question 20

Steps Involved in Outbreak Investigation (9)

Answer 20

• Confirm outbreak and diagnosis
• Define a case
• Identify cases & obtain information
• Describe data collected and analyse
• Develop hypothesis
• Test hypothesis: analytical studies
• Special studies
• Communicate results
• Implement control measure

Question 21

Multi Locus Sequence Typing (MLST)

Answer 21

Characterisation of bacterial isolates based on the combination of alleles at multiple loci (gene fragments)

Question 22

Whole Genome Sequencing (WGS)

Answer 22

• Characterisation of isolates based on unique whole
  genome sequences
• There is some debate about how to interpret WGS in
  their entirety

Question 23

Examples of Epidemic Curves: Common Point Source

Answer 23

• Commonly associated with foodborne illness, where
  many people are exposed for a short period of time
• Note lack of person-to-person transmission

Question 24

Examples of Epidemic Curves: Common Persistent Source

Answer 24

Outbreak due to exposure of persons to a noxious influence that is common to the individuals in the group

Question 25

Examples of Epidemic Curves: Propagated Source

Answer 25

An epidemic that arises when a primary case introduces an infectious agent to a population and subsequent host-to-host transmission results in ‘waves’ of cases

Question 26

Lab confirmation of Campylobacter takes….hours?

Answer 26

96